The invention relates to an electric incandescent lamp comprising:
a tubular sealed glass envelope; PA1 a helically coiled tungsten filament having turns, accommodated within the envelope, PA1 the filament having end portions which are connected to current conductors passing through the envelope to the outside, PA1 the filament being supported between its end portions by at least one constriction made in the glass envelope at an area where turns of the filament are short-circuited by a member of refractory metal.
Such an electric incandescent lamp is known from EP 0 446 458 A2.
The filament must be supported between its end portions in order to prevent the filament from approaching the envelope too closely or even touching it. A local overheating of the envelope is thereby prevented. Supporting the filament also results in the possibility to align the filament with respect to a reflector in which the lamp is used.
Wire-wound supports are present on the filament in most incandescent lamps to support the filament. These supports are applied to the filament when the coiling mandrel onto which the filament was coiled, is still present. The supports have a helically coiled portion about the filament, and a spiralling portion engaging the envelope. The filament must then be processed in order to remove the winding mandrel by etching and to secure current conductors. The processing of the filaments provided with their supports is cumbersome, however, because of the projecting supports. The supports of the filaments may, moreover, hook into one another, necessitating their disentanglement.
According to EP 0 446 458 A2, the filament may be supported by constrictions in the envelope directly engaging the filament in quartz glass lamps consuming a low power of up to 75 W. In quartz glass lamps consuming a higher power the constriction may touch a spacer in the filament. A spacer may be a single-coiled section or a non-coiled section of the filament between two coiled sections. Such intermediate sections consume less power per unit of filament length, have a lower temperature and may therefore be in contact with a quartz glass envelope without causing damage to the envelope.
It is said to be advantageous to have a core rod present in the filament at the area where it is in direct contact with the envelope. The core rod short-circuits turns of the filament, thereby lowering their temperature.
It is cumbersome, however, to maintain part of the winding mandrel as a core rod in the filament because to that end the filament must be coated locally with a wax to protect the mandrel against the etching liquid before etching of the mandrel, and the wax must be removed later on. As the mandrel must be dissolved and the filament must be conserved, the mandrel has to be of a different metal, usually molybdenum. It is a disadvantage, however, that the presence of this other metal in the completed lamp cannot be avoided in lamps of this construction. In principle, it would be possible to introduce a core rod into a finished filament, but this requires a lot of manipulation and involves the risk of filament distortion.